Metal oxide film structure

ABSTRACT

The invention provides metal oxide film structures for use as radioactive radiation windows, membranes of electrolytic cells and other purposes. The structures consist of an annular, or other closed frame of tantalum, niobium, tungsten or zirconium integral with a thin film of an oxide of the same metal, which spans the area enclosed by the frame. The structure is manufactured by subjecting a blank of the metal concerned, centrally masked on the larger part of the surface, to an anodizing treatment producing the oxide on all not-masked parts, removing the mask and subjecting the oxide-covered blank, as anode, to an electrolytic process by which the not-oxidized metal except the marginal portion, which constitutes the frame, is dissolved.

[451 July4, 1972 [54] METAL OXIDE FILM STRUCTURE [72] Inventor: Abraham Aladjem, Holon, Israel [73] Assignee: State of Israel, Atomic Energy Commission, Beer-Sheba, Israel 221 Filed: Sept. 19, 1969 [21] Appl.N0.: 859,310

3,510,656 5/1970 Hood ..250/84 FOREIGN PATENTS OR APPLICATIONS 1,143,376 2/1963 Germany ..204/143 Primary Examiner-Archie R. Borchelt AttorneyBrowdy and Neimark [57] ABSTRACT [30] Foreign Apphcauon Pnomy Data The invention provides metal oxide film structures for use as Sept. 26, 1968 Israel ..30770 radioactive radiation windows, membranes of electrolytic cells and other purposes. The structures consist of an annular, [52] US. Cl ..250/I06 S, 204/15, 204/32 R, 0 other closed frame of tantalum, niobium, tungsten or zir- 204/143 250/84 conium integral with a thin film of an oxide of the same metal, [5 It. CI. an the area enclosed the frame The tructure is [58] Field 01 Search ..250/84, 106 S; 204/15, 32 R, manufactured by subjecting a blank f the meta] concerned, 204/33 143 centrally masked on the larger part of the surface, to an anodizing treatment producing the oxide on all not-masked [56] Reierences cued parts, removing the mask and subjecting the oxide-covered UNITED STATES PATENTS blank, as anode, to an electrolytic process by which the notoxidized metal except the marginal portion, which constitutes 2,992,726 7/1961 Simens ..250/106'S X h frame, i di l d 3,332,859 7/1967 Dunn et al.. ..204/32 R 3,488,502 1/1970 Dukes ..250/106 S 13 Claims, 9 Drawing Figures EK i METAL OXIDE FILM STRUCTURE This invention relates to a metal oxide film structure in which the film consists of the oxide of one of the metals tantalum, niobium, tungsten or zirconium.

Films of these oxides have been used, for example, in electrolytic capacitors. For this use the oxide film has been produced as a coating on a support of the same metal, e.g., by anodizing the metal foil. All these known metal oxide films are supported by the metal support over their entire area.

This invention has the object to provide metal oxide film structures including a thin film of an oxide of one of the metals Ta, Nv, W or Zr, in which a substantial part of the area of the oxide film is unencumbered by the support.

In connection with this invention the term thin film means a coherent film of a thickness of the order of magnitude of 10 to 10 A., not exceeding about 10,000 A., which is impermeable to uncharged gas molecules but permeable to radiation, especially radioactive radiation including a-particles, B-particles and 'y-rays.

The absence of a support over a large area of the oxide film enables these metal oxide film structures to be used for various purposes where the use of a metal oxide film is most advantageous but such films could not be used up to now owing to the presence of the metal support over the entire area.

The metal oxide film structures of the invention are, for example, excellent radiation windows of capsules containing radium or other radioactive substances, instead of the mica films used hitherto for this purpose, which are more expensive to make and less resistant to the deleterious efiects of the radiation.

In end-window radiation counters the metal oxide film structures according to the invention can advantageously be used as windows instead of the conventional aluminized polyester films. In this case, too, the metal oxide film is much more resistant to the effects of the radiation than the polyester film.

In electrolytic cells the metal oxide film structures according to the invention can, with great advantage, replace the conventional parchment membranes, e.g., in the anodic oxidation of organic substances. 7

The invention consists in a metal oxide film structure comprising a thin film of an oxide of a metal selected from the group consisting of tantalum, niobium, tungsten and zirconium, which film is integral at its edges with a rigid frame of the metal of which the film is an oxide, and freely spans the space enclosed by said frame.

Thus, the invention includes, for example, Ta O films on tantalum metal frames; Nb,o, films on niobium metal frames; W films on tungsten metal frames (the oxide W 0 is nonstoichiometric); and ZrO films on zirconium metal frames.

The metal oxide film will as a rule be plane. However, the invention also contemplates other shapes, e.g., that of a convex-concave spherical segment.

The invention also provides a method for the manufacture of metal oxide film structures of the kind set out above, which comprises the steps of: providing a disc-shaped blank of a metal selected from the group consisting of Ta, Nb, W and Zr; applying to one surface of the blank a mask in a central portion of said surface; submitting the masked blank to a first electrolytic operation at a voltage in the range from 6 to 320 volts for producing a thin oxide film on the not-masked surface and edge areas of the blank; removing the mask from the blank, submitting the blank, connected as anode, to a second electrolytic operation at a voltage which is in the range from I to 20 volts and does not exceed about one-fifth the voltage used in the said first electrolytic operation; and continuing the second electrolytic operation until all the metal not oxidized in the first electrolytic operation is dissolved from the unmasked area of the previously masked surface of the blank through the thickness of the blank until the inner side of the metal oxide film at the opposite surface of the blank.

In the first electrolytic operation, which is a conventional anodizing process, a coherent metal oxide film is formed on the whole of the not-masked surface of the blank, on the edge of the blank, and on the not-masked marginal part of the masked surface. The metal enclosed between the marginal parts of both surfaces and the edge of the blank is not attacked in the second electrolytic operation but remains intact in the form of a frame integral with the oxide film.

The blank may have any desired suitable shape, e. g., circular or rectangular, according to the contemplated use of the metal oxide film structure. Of course, the frame retains the form of the blank. Thus, a circular blank produces an annular frame, a square blank a square frame, and so forth. Where the oxide film is to be plane, both surfaces of the blank are plane and parallel. Where, for example, a spherical-segment film is desired, one surface of the blank (the one which is not masked in the process) will be cambered accordingly,

It is found that a particularly advantageous electrolyte for use in the second electrolytic operation is a saturated aqueous-methanolic metal or ammonium halide solution where water content does not exceed 10 percent by weight. The metal halide is preferably an alkali metal or ammonium chloride.

The mask may be, for example, a liquidimpermeable piece of adhesive tape, teflon or the like. Any other masking means may be used instead, e.g., an O-ring of an adequately elastic material resistant to the electrolyte, enclosing the area of the blank to be masked and tightly pressed against the surface of the blank by any suitable means.

Since the blank, in its use as an anode in both electrolytic operations, has to be held and to be connected with the current leads, a preferred feature of the invention provides to make the blank initially integral with a tail or stem laterally projecting from the edge, which serves as a holder and a terminal. This will be cut off at the edge of the blank after the second electrolytic process. The cutting scar remains bare, i.e., not covered by the oxide film, but this fact does not interfere with any contemplated use of the structure since the scar is located on the edge while all uses of the structure concern the unsupported area of the oxide film in the area inside the frame.

The accompanying drawings illustrate the invention by way of example.

FIG. 1 is a side view of a blank with tail according to either FIG. 2 or 3;

FIG. 2 is a plan view of a circular blank;

FIG. 3 is a plan view of a square blank;

FIG. 4 shows the underside of the blank of FIG. 2 with a mask applied thereto;

FIG. 5 is a section, on line V-V of FIG. 4, of the same blank after the first electrolytic operation;

FIG. 6 is a view, similar to FIG. 5, of the same blank after the removal of the mask;

FIG. 7 shows similarly the same blank after the second electrolytic operation;

FIG. 8 shows similarly the finished metal oxide film structure;

FIG. 9 is an axial section of a radium capsule whose window is formed by the metal oxide film structure of FIG. 8.

The blank of FIG. 1 is a disc 1 made integrally with a tail 2. FIG. 2 shows a disc la which is circular. The tail projects laterally from the edge in a radial direction. In the case of a square disc lb, as shown in FIG. 3, the tail projects at right angles from the middle of one of the side edges.

The blanks of FIGS. 1, 2 and 3 are made of one of the metals tantalum, niobium, tungsten or zirconium.

FIG. 4 shows a blank according to FIG. 2 with a circular mask 3 applied to one of the surfaces of the disc. The mask is slightly smaller in diameter than the disc itself and arranged concentrically in regard to the disc. The mask may consist, for example, of an adhesive tape or teflon. It must be impermeable to the electrolytes used in the electrolytic operations.

By the first electrolytic operation the disc of FIG. 4 is covered with an oxide film comprising a part 4a covering the whole not-masked surface, a part 4b covering the edges, and a part 4c covering the not-masked marginal'part of the masked surface. The parts 40, 4b and 4c of the film from a coherent whole.

The tail 2 is not covered by an oxide film since it stays clear of the electrolyte.

In the stage illustrated by FIG. 6 the mask 3 has been removed and the blank is then submitted to the second electrolytic treatment. This dissolves a circular disc of not-oxidized metal whose surface area is identical with the unmasked area of the previously masked surface while its thickness is the same as that of the remaining not-oxidized metal part. That is, the dissolution of the metal goes through to the inner side of the surface part 4a of the oxide film. The edge portion la of the metal enclosed between the oxide film parts 4a, 4b and 40 remains unaffected by the electrolytic operation and forms a ring which constitutes a structural support for the metal oxide film.

Finally, the tail 2 is cut off, whereby the finished structure of FIG. 8 is produced. At the place where the tail 2 has been severed from the edge, there remains a scar, i.e., blank metal not covered by the metal oxide. The scar cannot be seen in FIG. 8.

FIG. 9 illustrates the use of a structure according to FIG. 7, as the window of a radium capsule. The capsule comprises a metal block 5 with a cavity 6 which accommodates the radioactive preparation 7. The cavity is covered with a structure according to FIG. 8, which has been referred to generally by the arrow 8. The structure is tightened against the capsule by any suitable conventional means which have not been illustrated since they are well known.

The invention is further illustrated by the following examples.

EXAMPLE I A tantalum blank 0.08 mm thick and 2.5 mm in diameter and integrally made with a tail of 1.2 mm length (substantially corresponding to FIG. 2) is degreased by flushing with trichloroethylene, then immersed for 30 minutes in a concentrated aqueous CrO -I-I SO solution and thereafter rinsed with a large amount of water.

The mask is applied to the disc (FIG. 4) and the disc is connected, by means of its tail 2 which serves as a holder and terminal, as the anode in an electrolytic circuit which comprises a platinum cathode and an electrolyte constituted by a 3 percent aqueous boric acid solution. A d.c. potential is applied with a current density of milliamperes/cm and a voltage of 200 volts. At this magnitude the voltage is set constant and the current density is allowed to drop to below 0.1 milliampereslcm By this treatment, a coherent tantalum oxide (Ta O film 4a, 4b, 4c is formed in the not-masked parts of the disc (FIG. 5).

Then the blank is removed from the electrolyte, the mask 3 is removed (FIG; 6), and the blank is connected as the anode in a circuit having a platinum cathode and an electrolyte constituted by a solution of ammonium chloride in aqueous methyl alcohol (water content 3 percent by weight). A current density of 50 milliamperes/cm at a voltage of 10 volts is applied, and the disc is left in the electrolyte until all the metal except the ring enclosed by the parts 4a, 4b, 4c of the oxide film has dissolved (FIG. 7). There remains merely a ring 10 of not-oxidized metal which forms a structural support of the tantalum oxide film 4a, 4b, 4c. The thickness of the film is about 3,000 A.

EXAMPLE 2 A niobium blank of the same size and shape as the tantalum blank of Example 1 is subjected to the same treatment as described in Example 1.

In the niobium oxide film structure thus produced the niobium oxide (Nb O film has a thickness of about 5,400 A.

EXAMPLE 3 A tungsten blank of the same size and shape as the tantalum disc of Example 1 is subjected to the same treatment as described in Example 1. i

In the tungsten oxide film structure thus produced the tungsten oxide (W0 0 not stoichiometric) film has a thickness of about 4,000 A.

EXAMPLE 4 A zirconium disc of the same size and shape as the tantalum disc of Example 1 is subjected to the same treatment as described in Example 1.

In the zirconium oxide film structure thus produced the zirconium oxide (ZrO film has a thickness of about 4,000 A.

I claim:

1. A metal oxide film structure comprising a thin film of an oxide of a metal selected from the group consisting of tantalum, niobium, tungsten and zirconium, which film is integral at its edges with a rigid frame of the metal of which the film is an oxide, and freely spans the space enclosed by said frame; the oxide film having a thickness not exceeding an order of magnitude of about 10 A. and being permeable to radioactive radiation but impermeable to electrically uncharged gas molecules.

2. A radioactive device comprising a capsule containing a radioactive substance, and a radiation window constituted by a metal oxide film structure according to claim 1.

3. A structure according to claim 1, wherein the thickness of the film is of the order of magnitude from 10 to 10 A.

4. A structure according to claim 1, wherein the frame is annular.

5. A structure according to claim 1, wherein the frame is rectangular.

6. A structure according to claim 1, wherein the frame consists of tantalum metal and the film consists of tantalum oxide.

7. A structure according to claim 4, wherein the tantalum oxide film has a thickness of about 3,000 A.

8. A structure according to claim 1, wherein the frame consists of niobium metal and the film consists of niobium oxide.

9. A structure according to claim 8, wherein the niobium oxide film has a thickness of about 5,400 A.

10. A structure according to claim 1, wherein the frame consists of tungsten metal and the film consists of tungsten oxide.

11. A structure according to claim 10, wherein the tungsten oxide film has a thickness of about 4,000 A.

12. A structure according to claim 1, wherein the frame consists of zirconium metal and the film consists of zirconium oxide.

13. A structure according to claim 12, wherein the zirconium oxide film has a thickness of about 4,000 A. 

2. A radioactive device comprising a capsule containing a radioactive substance, and a radiation window constituted by a metal oxide film structure according to claim
 1. 3. A structure according to claim 1, wherein the thickness of the film is of the order of magnitude from 102 to 103 A.
 4. A structure according to claim 1, wherein the frame is annular.
 5. A structure according to claim 1, wherein the frame is rectangular.
 6. A structure according to claim 1, wherein the frame consists of tantalum metal and the film consists of tantalum oxide.
 7. A structure according to claim 4, wherein the tantalum oxide film has a thickness of about 3,000 A.
 8. A structure according to claim 1, wherein the frame consists of niobium metal and the film consists of niobium oxide.
 9. A structure according to claim 8, wherein the niobium oxide film has a thickness of about 5,400 A.
 10. A structure according to claim 1, wherein the frame consists of tungsten metal and the film consists of tungsten oxide.
 11. A structure according to claim 10, wherein the tungsten oxide film has a thickness of about 4,000 A.
 12. A structure according to claim 1, wherein the frame consists of zirconium metal and the film consists of zirconium oxide.
 13. A structure according to claim 12, wherein the zirconium oxide film has a thickness of about 4,000 A. 